Telecommunication systems are composed of various geographically separated nodes having one or more signals being transmitted and received between nodes. For example, a cellular telephone system is composed of towers where each has a base station that transmits and receives RF signals to one or more cellular telephone transceivers. Signals transmitted over a radio link may be attenuated due to such factors as propagation loss and multipath fading. Since the amplitude of the signal is attenuated during transmission between nodes, communication signals typically require power amplifiers (PAs) to compensate for these losses.
It is desired that a PA produce a linear output so that the amplifier accurately reproduces the signal present at the input in both amplitude and phase. Therefore, an ideal PA will pass the input signal through to the output undistorted but enlarged with a gain set by the user and with no delay, independent of the output impedance of the input signal source. In addition the ideal PA will be able to drive any load; i.e., supply any current. In reality, however, PAs are not ideal over their entire operating range. A PA that does not have a linear input/output relationship will cause unwanted amplitude variations of the output signal (e.g., spreading unwanted harmonics onto adjacent radio frequencies), which may interfere with other radio channels. Third-generation (3G) cellular wireless communication systems, for example, have a need for high linearity at the PA output to achieve a high adjacent channel leakage ratio (ACLR) and a low error vector magnitude (EVM).
To suppress unwanted PA nonlinearity, predistortion circuits have been made and used. A predistortion circuit models the PA's gain and phase characteristics and provides an output signal, when combined with the PA's input signal, produces an overall system that is more linear (in reference to the unpredistorted input signal). Thus, distortion or predistortion is purposely introduced into the input signal of the PA with the goal of correcting any non-linearity in the output signal of the PA. In some implementation of the pre-distortion circuit, there is another goal, which is to provide a memoryless output signal. One example of a predistortion apparatus that can be used as a linearizer for a PA for RF applications is disclosed in U.S. patent application Ser. No. 11/484,008, entitled “Pre-Distortion Apparatus,” filed on Jul. 7, 2006.
In addition to causing a PA to provide a linear output signal, another advantage of using a pre-distortion circuit is added cost savings. As power increases to its maximum rated output, a PA without any predistortion tends to have a non-linear output that becomes progressively worse as the maximum rating is approached. Thus, predistortion obtains more usable power from the PA, without resorting to a larger and more expensive device.
Various pre-distortion techniques have been described in the prior art. Some devices use digital predistortion logic circuits which use data stored in a look-up table containing a “mirror image” of the characteristics of the measured signal. Alternatively, these “mirror image” characteristics may be preprogrammed into predistortion components operating in the RF circuitry in a technique known as “analog feed-forward.” Yet another predistortion technique is known as “polynomial-based” digital predistortion (DPD), which entails digitally predistorting a signal at baseband using polynomial basis functions. With the appropriate feedback, time-varying PA characteristics can be optimally adjusted using DPD.
Although DPD is widely used today, DPD solutions suffer from the problem of high power consumption and high cost because nonlinear predistortion expands signal bandwidth by a factor of five or more. This problem is a critical issue in a commercial cellular wireless system governed by in-band and out-of-band specifications for base stations communicating with mobile telephones and for repeaters used to extend base station coverage. In base station and repeater applications, it is often too expensive to take the conventional approach, which requires RF-to-digital down-conversion and digital-to-RF up-conversion before and after DPD, respectively. Another problem with DPD is in its application to medium-to-low power (e.g., 10 W) PAs, such as, for example, PAs used in beam-forming antenna arrays. In antenna arrays, using DPD on each PA in the array can severely limit the overall energy efficiency of the entire system. Power usage and unit cost become a significant concern for companies deploying cellular telephone networks with millions of base stations and repeaters.
The present disclosure describes a novel linear power amplifier providing superior performance by using an analog RF predistortion block for distortion of RF signals.